Spatial and Temporal Instabilities of Optical Discharges

Abstract

By now technologies, employing optical discharges find ever expanding applications in metering and diagnostic equipment in science, engineering and medicine. Based on the original experimental results authors look into some manifestations of spatial and temporal instabilities of the continuous and periodic pulse optical discharges (COD, POD). Set of the phenomena considered makes a great impact on a performance of laser produced plasmas essential for many applications, such as high brightness broadband light sources, for instance. Performance instability of continuous optical discharges followed by laser beam refraction on the refraction index gradients exhibit themselves first in spatial inhomogeneity of plasma thermal radiation luminosity and the other parameters related. Spatial inhomogeneity is accompanied by temporal instability of the plasma. The paper reports criteria for the appearance of instabilities related to the refraction followed by the limitations on supporting of elongated plasma. One of the main reasons of temporal instabilities of COD is thermal gravity convection. Instability of a heated gas zone surrounding optical discharge is followed by regular self-sustained oscillations leading in turn to pulsing of brightness and position of radiated plasma. Simple physical model proposed gives estimations correspondent to the observed pulse frequency dependence on gas pressure. In the case of periodic pulse optical discharges forced convection may be put forward as one of the main discharge instability reasons. Pulse optical discharge induces convective flows due to asymmetrical gas expanding following gasdynamical effect of the energy release zone shape determined by laser beam focusing system configuration.

Oscillations of convective flow around a continuous optical discharge in high-pressure xenon

“…In [11], the study of relatively small (about 1%) pulsations of COD plasma brightness was carried out. Similar pulsations have been observed in broadband emission sources based on COD [9]. It was found that the observed oscillations are of a physical nature.…”

“…The method of obtaining shadow patterns was similar to that used in [11]. A laser-plasma radiation source LPS-50 [9] was used as a point-like light source for the shadowgraphy device. A collimated beam from the point-like source passed through the bulb with plasma and was then focused by an objective that produced a shadowgraphy image in the matrix of the CCD registration camera.…”

“…The COD was sustained in a scheme of crossed laser beams. The same optical scheme is used in the design of the laser-plasma emission source LPS-50 [9]. Radiation from both PLD-40-type laser diode modules was directed into a closed quartz bulb with high-pressure xenon by means of a collimator and a focusing lens.…”

“…In recent decades, COD plasma technology has been applied to high-brightness broadband laser-plasma radiation sources [6][7][8][9][10]. One possible design of the radiation source head is shown in figure 1.…”

“…One possible design of the radiation source head is shown in figure 1. One of the main requirements for laser-plasma radiation sources is the high temporal and spatial stability of plasma and radiation emission [9][10][11]. Nevertheless, even under complete stabilization of the input parameters (laser power and gas pressure), high-temperature gradients in COD induce convection in the plasma-forming gas followed by pulsations of gas density, temperature and velocity that negatively affect the stability of plasma itself [11].…”